This invention relates generally to a method and apparatus for controlling a variable displacement hydraulic pump and, more particularly, to a method and apparatus for controlling nonlinear characteristics associated with the pump discharge pressure of a variable displacement pump.
Variable displacement hydraulic pumps are used in a variety of applications. For example, hydraulic construction machines, earthworking machines, and the like, often use variable displacement hydraulic pumps to provide the pressurized hydraulic fluid flow required to perform desired work functions.
Operation of the pumps, however, is subject to variations in pressure and flow output caused by variations in load requirements. It has long been desired to maintain the pressure output of the pumps in a consistent manner so that operation of the hydraulic systems is well behaved and predictable. Therefore, attempts have been made to monitor the pressure output of a pump, and control pump operation accordingly to compensate for changes in loading.
For example, U.S. Pat. Nos. 4,510,750 and 5,865,602, to Izumi et al. and Nozari, respectively, disclose the use of feedback systems which monitor characteristics such as pump output pressure, and provide feedback control of the pump in attempts to operate the pump in a desired manner. However, neither Izumi et al. nor Nozari account for the wide range of nonlinearities inherent in hydraulic pump operation. The disclosed patents of Izumi et al. and Nozari are limited to a linear range of pump operation in which behavior of the pump is fairly predictable and thus may be controlled using well known linear control techniques.
Nonlinear control methods exist which may be used to control systems having essentially nonlinear behavior, such as variable displacement pumps. For example, one of the most common methods of control is to first linearize a nonlinear system and then control the resultant linear system. A common example of such a system involves a Taylor Series linearization, which linearizes a small portion of the system about an operating point, the portion being essentially linear in nature to begin with. The drawback of a method such as this is that predictable performance is only assured if the system stays close to the particular point about which it is linearized.
Another method is to use a technique commonly known as gain scheduling, in which a series of operating points are selected, then a small portion about each operating point is linearized, e.g., by a method such as the Taylor Series. However, this results in a discrete system which does not function well as the system moves from one operating point to another.
A method known as feedback linearization may be used to transform nonlinear dynamics of a system to linear equations, which may then be used to control the system in an effective manner. For example, in U.S. Pat. No. 5,666,806, Dietz discloses a system which uses feedback linearization control laws to control the nonlinear behavior of a hydraulic system, in particular the nonlinear behavior of a hydraulic cylinder. However, the system disclosed by Dietz incorporates nonlinearities from multiple sources, such as a pump, cylinder, control valve, and the like. As a result, Dietz is required to apply feedback linearization control laws to many sources of nonlinearities, thus resulting in linearized equations having multiple, i.e., fourth order, dynamic response characteristics.
In the present invention, it is desired to control a single device, i.e., a variable displacement hydraulic pump, within a hydraulic system, and thus control the nonlinear characteristics associated with the hydraulic pump. It is also desired to control the pump using feedback linearization control laws to control the discharge pressure of the pump over a wide range of nonlinear operating conditions. Furthermore, it is desired to control the nonlinear characteristics of the pump using feedback linearization control laws which create a first order system tracking response, thus providing control over nonlinearities without overshoot for step response.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention a method for controlling a pump discharge pressure of a variable displacement hydraulic pump having a swashplate, and a servo valve for controlling an angle of inclination of the swashplate, is disclosed. The method includes the steps of sensing a value of an actual pump discharge pressure, determining a desired control pressure using a first feedback linearization control law, determining a desired servo valve spool position using a second feedback linearization control law, and controlling the value of the actual pump discharge pressure as a function of the first and second feedback linearization control laws.
In another aspect of the present invention an apparatus for controlling a pump discharge pressure of a variable displacement hydraulic pump is disclosed. The apparatus includes a swashplate inclinably mounted to the pump, a servo valve hydraulically connected to the pump for controlling an angle of inclination of the swashplate, a pump discharge pressure sensor connected to an output port of the pump, and a controller electrically connected to the pump for sensing a value of an actual pump discharge pressure, determining a desired control pressure using a first feedback linearization control law, determining a desired servo valve spool position using a second feedback linearization control law, and controlling the value of the actual pump discharge pressure as a function of the first and second feedback linearization control laws.